1. Field of the Invention
The invention relates to a receiver for optical heterodyne or homodyne detection of an optical signal beam, which receiver comprises: a local oscillator for producing a beam of radiation at least one polarization-sensitive beam splitter for splitting the signal beam into two orthogonally polarized components, at least one polarization-insensitive beam-combining element for combining radiation from the signal beam with radiation from the local oscillator beam, and a detection system for converting the combined radiation into at least one electric signal suitable for further processing.
2. Description of the Related Art
Receivers for optical heterodyne detection are used in optical signal transmission. By mixing the signal beam in a heterodyne detector with an optical beam emitted by a local oscillator, a considerably better result is obtained in signal-to-noise ratio and discrimination of against background radiation as compared with direct detection of the signal beam.
The principle of heterodyne detection of optical radiation is extensively described in the Article "Optical Heterodyne Detection" by O. E. DeLange in the journal "IEEE Spectrum" of Oct. 1968, pp. 77-85. As has been stated in this Article, it is important that the states of polarization of the signal beam and the local oscillator beam correspond as closely as possible. A possible solution to achieve this is to split the signal beam into two sub-beams having mutually perpendicular and fixed directions of polarization. The two sub-beams are then each combined with local oscillator radiation which is polarized in the same direction.
In principle, four components are required for splitting and combining the beams: two polarization-sensitive beam splitters for splitting the signal beam and the local oscillator beam, respectively, and two polarization-insensitive beam-combining elements for combining the sub-beams formed.
By firstly combining the signal beam with the local oscillator beam and subsequently splitting it by means of a polarization-sensitive beam splitter into two orthogonally polarized sub-beams, one of the beam-combining elements can be dispensed with. Since a beam-combining element not only has two inputs but necessarily also two outputs, two polarization-sensitive beam splitters remain necessary to absorb and detect all signal radiation.
The beam-combining element used in practice in a heterodyne receiver is a fibre-optic polarization-maintaining directional coupler. Such a coupler comprises two optical waveguides which run parallel at a short distance from each other over a length of approximately five centimeters. Both ends of each waveguide are coupled to an optical fibre having a length of several dozen centimeters, the so-called pigtails. The ends of the pigtails are coupled to further optical fibres for applying the signal beam and the local oscillator beam and for further guiding the combined beams to the detection system. Such a system requires a fairly large space, not only because of the required relatively large length of the directional coupler but particularly because of the length of the pigtails. It is true that the latter can be curled but the radius of curvature should not be much less than about ten centimeters so that the linear dimension of the optical part of the heterodyne detector is at least approximately twenty centimeters.
A further drawback of such a system is that losses occur in the couplings between the various optical components and optical waveguides, which couplings thus attenuate the signal to be transmitted.